Preselector circuit for high frequency receivers



Sept. 10, 1935. W.-F. CURTIS 2,013,650

PRESELECTOR CIRCUIT FOR HIGH FREQUENCY RECEIVERS Filed June 22, 1934 T0 RECEIVER 17 .1 T Z0 Z6 ,7/ 7\?\ 11! l l l I Fiy. 1

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INVENTOR wesil y F. Curtis Patented Sept. 10, 1935 PRESELECTOR CIRCUIT FOR I-HGH FREQUENCY RECEIVERS Westley F. Curtis, Beaver Heights, Md.

Application June 22, 1934, Serial No. 731,869

2 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) or none at all.

It is often found desirable in receiver design to employ some sort of pre-selector; that is, to couple the antenna to the receiver through a network which strongly attenuates undesired signals and attenuates the desired signal very little. This is ordinarily done by connecting one or more parallel-tuned circuits in shunt with the in put of the receiver. The efiiciency of this sort of pre-selection increases markedly with decreasing resistance in the tuned circuits; decreased resistance results not only in increased attenuation of undesired signals, but also in decreased attenuation of the desired signal. The difficulty involved in designing high frequency receivers of great selectivity is that the effective resistance generally increases when the frequency is raised.

The primary object of this invention is to provide an improved pre-selector system, the output of which may be connected to a radio receiver in which the difficulty incident to the increase of the effective resistance with an increase of frequency may be minimized.

Another object of this invention is to provide a device having negative resistance characteristics, the value of the negative resistance of which may be varied without varying the coupling between the circuit including such negative resistance and another portion of the system.

With the above and other objects in view, the invention consists in the construction, combination and arrangement of parts as will be described more fully hereinafter.

A more detailed understanding of the present invention may be had from the following description taken in connection with the accompanying drawing, in which:'

Fig. 1 is a schematic circuit diagram showing one form the present invention may take; and.

Fig. 2 is a characteristic curve showing the relation between the voltage applied to and the current between the cathode and plate anode of the tube included in Fig. 1.

The system shown in Fig. 1 includes an antenna I connected to ground at 3 by Way of the primary coil 2 of a radio frequency transformer. The secondary coil 4 of the radio frequency transformer is shunted by a tuning condenser 5 which together with the coil 4 constitutes a tuned resonant circuit that may be tuned to any frequency that it is desired to pre-select. The coil 4 is connected to ground tat one end by way of the condenser H. The other end of coil 3 is connected to the plate electrode ID of thermionic tube 6 and to the lead 22 to the receiver by way of condenser I9. A difference of potential is main- 5 tained between the cathode l and the plate electrode iii of the thermionic tube 6 by means of a battery II or other source of potential. The negative terminal of the battery H is connected to the cathode l and the whole of battery H is 10 shunted by a resistor It. A slider 20 connected to coil 4 is provided so that any desired potential may be applied between the cathode l and the plate electrode Ill, The battery or other source of potential 12 has its negative terminal 15 connected to the positive terminal of the source of potential H and the positive terminal of I2 is connected to the screen like anode 9 of the thermionic tube 6. The screen-like anode 9 is also connected to ground 3 by way of the condenser IS. The grid or control element 8 of tube 6 is connected by way of resistance l8 to the slider 2| associated with the resistance l5 connected across battery l3; By means of this arrangement it is possible to apply any desired potential between cathode l and grid 8. Lead 23 connected to cathode 1 and ground 3 forms together with lead 22 the output leads of the pre-selector or the input leads to the radio receiver 7 The coil 2 transfers the radio frequency energy intercepted by the antenna system to the tuned system comprising coil 4 and condenser 5 which is tuned to the frequency it is desired to select and apply to the receiver by way of leads 22 and 23. To overcome the inherent resistance in the tuned circuit comprising elements 4 and 5 and render said circuit sharply resonant to the desired frequency there is connected in shunt with said circuit an arrangement adapted to produce an equivalent series negative resistance in the tuned cir- 49 cuit. That is, there is connected in shunt with the tuned circuit, a device in which the current will decrease as the potential applied thereto increases. This is accomplished by using what is commonly known in the art as a dynatron tube. The action of the dynatron may be better understood by reference to the curve shown in Fig. 2 of the drawing wherein the ordinates represent the total currents flowing to or from the plate electrode Iii of the tube shown in Fig. 1 for 50 varying values of applied potential represented as abscissae. When the tube 6 is adjusted to operate on that portion of its characteristic curve between points B and C, and the potential of .the grid 8 is adjusted by means of pointer 2| and resistor IE to provide an equivalent negative resistance in the tuned circuit 4-5 at which the device just fails to oscillate, the resonant characteristics of the tuned circuit 4-5 are materially sharpened and all frequencies except the frequency to which the tuned circuit is adjusted are strongly attenuated, and that frequency attenuated little or none at all may be transferred to a suitable receiver (not shown) connected to leads 22 and 23.

As this circuit involves the use of a screen-grid tube, it might be of interest to compare its performance with the performance of a screen-grid amplifier stage. A screen-grid tube has a theoretical amplification factor which may be as large as 400 or even larger; at audio frequencies an over-all voltage amplification (gain) of as high as 200-300 may be obtained. However, due to the shunt admittances of the tube and circuit, the gain falls off rapidly as the frequency is increased. In the broadcast band a gain of 30-40 per stage is expected while at 20 megacycles experience indicates that a gain of 2 is exceptional. Above this frequency, little is known about the gain of R. F. amplifiers, but it cannot be above 2 and probably rapidly becomes smaller.

An ordinary screen-grid tube, such as the UX-222, will oscillate up to 25 me. when connected as a dynatron. Specially designed tubes would doubtless increase this range. Within this range it is possible to decrease the total effective resistance of the tuned circuit to very low values. For frequencies over 10 mc., the pre-selector circuit shown in Fig. 1 does not have a great deal less gain than a stage of amplification using the same tube.

In the case of the ordinary amplifier, negative resistance can be introduced in the tuned circuit only through the means of coupling to other parts of the circuit. As this is apt to lead to undesirable reactions in the receiver, little or no negative resistance can be used. It therefore follows that the pre-selector circuit shown in Fig. 1 has superior selectivity whenever an appreciable negative resistance can be obtained.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. In a pre-selector system a resonant circuit, means for imparting energy to said system at a plurality of frequencies including the frequency of said resonant circuit, means for deriving energy from said system, negative resistance means for sharpening the resonance characteristics of said circuit, comprising a thermionic tube having a cathode, a plate electrode, a grid-like anode intermediate said cathode and said plate electrode and a grid electrode between said cathode and said grid-like anode, a conductor extending between said plate electrode and one end of the inductance of said resonant circuit, a source of potential connected between the other end of said inductance and said cathode for maintaining the plate electrode at a higher potential than said cathode, a source of potential connected in circuit between said cathode and said grid-like anode for maintaining said grid-like anode at a higher potential than said cathode and said plate electrode, and adjustable means for applying to said grid electrode a potential for adjusting the value of the negative resistance characteristics of said tube.

2. In a pre-selector system, the combination of a tuned resonant circuit having positive resistance characteristics and means for rendering said tuned circuit sharply resonant comprising a dynatron device having a negative resistance characteristic connected so as to compensate at least in part for the positive resistance of the circuit, said dynatron device comprising a thermionic tube having a cathode, a screen-like anode, a plate electrode and a control electrode inclosed in an evacuated envelope and having sources of potential applied to said cathode, screen-like anode and said plate electrode in such a way that the device has a working range over which the current flow from the cathode to the plate electrode varies inv-ersely as the potential applied thereto, and means for applying a variable negative potential to said control electrode with respect to said cathode whereby the equivalent negative resistance between said plate electrode and said cathode may be varied.

WESTLEY F. CURTIS. 

